1. Field of the Invention
This invention relates to voltage controlled elements useful as voltage controlled amplifiers and/or potentiometers.
2. Description of the Prior Art
Multiplier circuits, amplifier circuits and attenuator circuits are disclosed in U.S. Pat. Nos. 3,714,462; 4,155,047; 4,157,512; 4,163,197; 4,169,247; 4,177,432, 4,180,780 and 4,187,472. Such circuits have also been disclosed in the article "Designers' Guide to: Basic AGC Amplifier Design" appearing in the Jan. 20, 1974 issue of Electronic Design News, the article "Voltage-Controlled Amplifier Covers 70 db Range" appearing in the Mar. 5, 1975 issue of Electronic Design News, the article "Linear Voltage-Controlled Attenuator-DC to 20 kHz" appearing in the March 1976 issue of Electronic Engineering, the article "A Monolithic Voltage-Controlled Amplifier Employing Log-Antilog Techniques" appearing in the March 1976 issue of the Journal of the Audio Engineering Society, the article "A Multiplex Remote-Control System" appearing in the September 1977 issue of the Journal of the Audio Engineering Society, the article "Current-Compensated Op Amp Improves OTA Linearity" appearing in the Mar. 17, 1977 issue of Electronics, in the paper entitled "Techniques for the Realization and Application of Voltage Controlled Amplifiers and Attenuators" presented at the Audio Engineering Society Convention in May of 1978 at Los Angeles, Calif. and in the paper entitled "The Design and Integration of a High Performance Voltage Controlled Attenuator" presented at the Audio Engineering Society Convention in November, 1979 in New York City.
Patents disclosing use of feedback to improve circuit performance include U.S. Pat. Nos. 4,163,948; 4,197,426 and 4,201,888. Use of feedback to improve circuit performance is also disclosed in the paper "Multipurpose Hardware for Digital Coding of Audio Signals" presented at the National Telecommunications Conference held December, 1977 in Los Angeles and in the paper "Electronically Tunable RC Sinusoidal Oscillators" published in the June, 1975 IEEE Transactions on Instrumentation and Measurement.
Also of interest is the product bulletin entitled "Engineering Data--Allison EGC-101 and Related Circuits" distributed by Valley People, Incorporated, P.O. Box 40306, 2028 Erica Place, Nashville, Tenn., 37204, disclosing substantially the circuits disclosed in the '462 patent.
The '047 and '462 patents are believed of greatest relevance of this invention.
The '047 circuit is a multiplier circuit accepting as input a differential input current and developing a differential output voltage proportional to the input current. Gain is determined by a control voltage establishing a bias on the transistor pairs, causing emitter current to split predictably. Various support circuitry may be used to convert a single input signal to a differential input current and to convert the differential outputs to a single signal.
The '047 circuit unfortunately requires extreme matching between left and right components. Without close matching, direct current appears in the output with the direct current component being dependent on the control voltage, i.e. control voltage feedthrough results. Moreover, thermal drifts can cause circuit output to drift unacceptably. A further disadvantage of the '047 circuit is the lack of log conformity in the gain control. This lack of conformity is particularly apparent near maximum gain. Still further, the maximum gain available with the central portion of the '047 circuit is unity. Gain can be introduced to the output stage, but, unfortunately, this leads to degradation in the signal-to-noise ratio.
An additional disadvantage of circuits of the '047 type is caused by noise of the central transistors; output noise may be sufficient that the equivalent input noise is unacceptably high, making such circuits inoperable with small signals.